In order to assess the significance of activated oncogenes in the pathogenesis of human gastrointestinal malignancies, especially gastric and hepatic carcinomas, fresh tumors of the stomach, rectum and liver were screened utilizing both DNA transfection and gene amplification techniques. In one of 35 stomach cancers, a threefold amplification of the K-ras gene was detected, and of five rectal cancers, one was positive in the DNA transfection assay. The oncogene detected in the latter was N-ras, with a 61st codon mutation.